1. Field of the Invention
The present invention relates generally to a self-diagnostic apparatus for diagnosing a meter mounted on an automotive vehicle, and more specifically to a self-diagnostic apparatus for diagnosing a vehicle meter (e.g. vehicle speed meter) actuated in response to a measurement signal detected by a vehicle sensor (e.g. vehicle speed sensor).
2. Description of the Prior Art
An example of prior-art speed meters mounted on an automotive vehicle is disclosed in Japanese Published Unexamined (Kokai) Utility Model Appli. No. 55-64773 for instance, which comprises a waveform shaper, an F-V converter, a cross coil meter, etc. That is, a vehicle speed pulse signal detected by a vehicle speed sensor is first waveform shaped and then converted into a voltage signal through the F-V converter. The converted voltage signal is applied to a cross coil composed of a pair of coils disposed a predetermined angular distance away from each other, to energize the cross coil, so that a movable magnet is rotated by the magnetic field generated by the cross coil. Vehicle speeds can be indicated by a needle fixed to the movable magnet on the basis of the angular position of the needle. To diagnose the vehicle speed meter as described above, Japanese Patent Appli. No. 61-123749 discloses the following method: a pseudo vehicle speed signal is applied to the vehicle speed meter, in place of an actual vehicle speed signal detected by the vehicle speed sensor, to confirm that a predetermined vehicle speed value can be indicated on the vehicle meter in response to the applied pseudo vehicle speed signal.
FIG. 1 shows a self-diagnosis apparatus disclosed in the above prior-art patent application, which comprises a vehicle speed sensor 1, a cross coil meter 3, an operational amplifier 5, a cross coil driver 7, a self-diagnose starter 9, a pseudo speed generator 11, etc. In operation, a vehicle speed signal detected by the vehicle sensor 1 is supplied to the cross coil meter 3 via the operational amplifier 5 and the cross coil driver 7. In self-diagnosis, a pseudo vehicle speed signal is generated from the pseudo vehicle speed signal generator 11. This signal generator 11 is activated by the self-diagnose starter when a diagnose switch 13 is turned on or the vehicle sensor 1 is disconnected. The generated pseudo vehicle speed signal is amplified by the operational amplifier 5 and then supplied to the cross coil meter 3 via the cross coil drive 7. On the basis of ths pseudo vehicle speed signal, the cross coil meter can be diagnosed by confirming whether a predetermined speed value can be indicated on the meter in response to the generated pseudo vehicle speed signal.
In the prior-art self-diagnose apparatus for a vehicle meter, however, there exists a problem in that in case the vehicle sensor 1 is disconnected by an accident during actual vehicle travelling, a vehicle speed determined on the basis of the pseudo vehicle speed sensor signal (which is different from an actual vehicle speed) is indicated, because a self-diagnosis mode is set when the vehicle speed sensor 1 is disconnected. The above vehicle speed indicated in response to the pseudo vehicle speed signal will result in driver's confusion.
In addition, in the prior-art self-diagnose apparatus for a vehicle meter, since the vehicle speed meter is often inspected in vehicle assembly process in the factor by disconnecting the vehicle sensor 1 from the meter to generate a pseudo vehicle speed signal, there exists another problem in that the inspector often forgets reconnecting the disconnected vehicle sensor to the meter after inspection.